An anti-lock brake system (ABS) is a device that prevents wheels from being locked. There is a known technique, disclosed in, for example, PTL 1, that determines whether the state (concavo-convex road surface) of a traveling road surface is bad in, for example, a motorcycle.
The technique in PTL 1 determines that the road state is bad when the number of times the wheel acceleration becomes equal to or more than a predetermined level (L1) is equal to or more than a predetermined number (KN1).
By the way, change of wheel speed during travel on a bad road is likely to vary depending on the magnitude of a braking power. However, since determination conditions are constant in the technique in PTL 1, further improvement of determination precision has been desired.